Add TSAN acquire/release calls to runtime/cgo to match the ones
generated by cgo.  This avoids a false positive race around the malloc
memory used in runtime/cgo when other goroutines are simultaneously
calling malloc and free from cgo.

These new calls will only be used when building with CGO_CFLAGS and
CGO_LDFLAGS set to -fsanitize=thread, which becomes a requirement to
avoid all false positives when using TSAN.  These are needed not just
for runtime/cgo, but also for any runtime package that uses cgo (such as
net and os/user).

Add an unused attribute to the _cgo_tsan_acquire and _cgo_tsan_release
functions, in case there are no actual cgo function calls.

Add a test that checks that setting CGO_CFLAGS/CGO_LDFLAGS avoids a
false positive report when using os/user.

Change-Id: I0905c644ff7f003b6718aac782393fa219514c48
Reviewed-on: https://go-review.googlesource.com/23492
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>